The term is just so damned vague. If I buy a stock and think the price will go down, is that "negative thinking"? If I'm miserable where I work, would it be "positive thinking" to tell myself that I like it there?

My current understanding is that "positive thinking" is a lot like "having an open mind": social gamesmanship where you say "I disagree with you" but in a way designed to make the other person look bad, at least in the eyes of people who don't both to find out what the words are supposed to mean.